ITEM:

INFORMATIONAL ITEMS/STAFF REPORTS

 

27.

SEMI-ANNUAL GROUNDWATER QUALITY MONITORING REPORT

 

Meeting Date:

February 28, 2008

Budgeted:

Yes

 

From:

David A. Berger,

Program/

Hydrologic Monitoring 2.6

 

General Manager

Line Item No.:

2-6-1 G, and 2-6-2 D

 

Prepared By:

Joe Oliver and

Cost Estimate:

N/A

 

Tom Lindberg

 

General Counsel Approval:N/A

Committee Recommendation: N/A

CEQA Compliance:N/A

 

SUMMARY:Water quality results from the Fall 2007 sampling of the Monterey Peninsula Water Management Districtís (Districtís) monitor well networks in the Carmel Valley aquifer and the Coastal Subareas of the Seaside Groundwater Basin are presented and briefly summarized below.

 

BACKGROUND:The District has maintained a groundwater quality monitoring program in the Carmel Valley Aquifer since 1981, and in the Seaside Groundwater Basin since 1990. Currently, collection of samples from the Carmel Valley monitor wells is conducted on an annual basis.The sampling schedule for Carmel Valley is staggered, with upper valley wells (i.e., upgradient of the Narrows), sampled in Spring and lower Carmel Valley wells in sampled Fall, to coincide with the historically higher nitrate concentrations in these respective areas.Collection of samples from the Seaside Basin monitor wells is conducted once per year in Fall, coinciding with the historically low water levels in the basin at this time of the year.It should be noted that, in 2007, the District was retained by the Seaside Basin Watermaster to collect water quality samples from the monitor wells in the Districtís Seaside Basin coastal monitor well network on a quarterly basis.The results of that sampling are reported to the Seaside Basin Watermaster Board on a quarterly basis.Also, coincidental with the work for the Watermaster, the District began using the services of a different, more convenient and more economical water quality analytical laboratory.Therefore, more chemical constituents are being analyzed and those data are presented for 2007.

 

DISCUSSION/ANALYSIS:

Carmel Valley Aquifer Monitor Wells - Results from the Fall 2007 sampling are provided in Exhibit 27-A.Eight monitor wells in the lower Carmel Valley were sampled during Fall 2007, as per the sampling schedule described above.The locations of these sampling points are shown on the map in Exhibit 27-C.Review of these water quality results indicates that, in general, there are minor changes in overall water quality compared to samples collected in Fall 2006 (data reported in the February 22, 2007 Board packet and provided in Exhibit 27-B).Staff is particularly interested in tracking indicators of potential seawater intrusion in the coastal portion of Carmel Valley.Accordingly, three sets of wells were established west of Highway 1, with each set being made up of three wells completed at different depths.Review of historical data indicated that the shallower and intermediate wells in the coastal area are subject to the mixing of fresh water and saline water as high tides and surf overtop the sand berm between the lagoon and the ocean.This contributes to episodic mixing within the shallower and intermediate zones of the aquifer, but is not indicative of larger-scale potential seawater intrusion into the aquifer.Currently, only the deeper wells at each of the three coastal locations are sampled.

 

Graphs showing historical specific electrical conductance (SEC) and chloride concentration in the deepest wells at each of the three coastal well sites are shown in Exhibits 27-E, 27-F, and 27-G.Note that the scales on the vertical axes of these graphs are different to help discern trends in the data.Well 16S/1W-14Jg is the deepest in the array of three wells located at the Carmel River State Beach parking lot at River Mile (RM) 0.07 (approximately 375 feet from the shoreline).Exhibit 27-E shows that SEC and chloride concentration slightly increased in this well in 2007, continuing a trend since 2005.Staff will continue to monitor this increasing trend however, it should be noted that these levels have not approached the levels observed at this location in the early 1990ís.These higher values observed early in the period of record at this site are at least partially attributable to the fact that there was no freshwater surface inflow to the lagoon for approximately four years (April 1987 until March 1991).This lack of freshwater inflow for local ground water recharge, combined with the proximity to the ocean and the permeability of the alluvial sediments, allowed for inland movement of the freshwater seawater interface past this site near the end of the 1987 Ė 1991 drought period.

 

Graphs of water quality data at the two coastal sites located farther from the shoreline show that SEC and chloride concentration did not appreciably change from 2006 to 2007 at either well (Exhibits 27-F and 27-G).At both sites, there appears to be a slight trend toward increased levels over the period of record.As noted in a prior report, the anomalously high SEC and chloride concentration in well 16S/1W-13Lc in 2000 are suspicious and may be attributable to sampling or analysis error (Exhibit 27-G).Additional background on historical water quality at the coastal monitor well sites can be found in District Technical Memorandum 90-04, Summary of Carmel Valley Groundwater Quality from Coastal Monitor Wells, which is available at the District office.†† Staff will continue to track future results for trends that might indicate significant changes in concentrations of these or other constituents in the coastal area of the aquifer.

For the five wells located farther inland, changes in SEC and chloride concentration did not vary significantly from the previous yearís sample results.The graph in Exhibit 27-H shows SEC and chloride concentration in well 16S/1E-23La, located at RM 6.72.The increased levels of SEC and chloride concentration that were observed in this well in 2005 had returned to below 2004 levels.The high chloride concentration in well number 16S/1E-23La in Spring 1993 is anomalous.Staff will continue to track future results for trends.

It was noted that one other well, 16S/1E-23E4, located at RM 6.53, yielded anomalous results for a number of constituents in 2006, most notably the concentration of iron.While the concentration of iron was still high in 2007, it should be pointed out that the concentration is nearly 75 percent lower than measured in 2006.As noted when results from 2006 were reported, due to the proximity of the wellhead at this site to the county road, it has been subject to periodic covering and flooding from urban runoff.It is believed that the well was contaminated by surface runoff prior to the Fall 2006 sampling, and that the well was not fully evacuated of the standard three casing volumes prior to sampling that year.Extra effort will continue to be employed at this site in the future to ensure the restoration of this well and the reliability of data acquired.

 

Seaside Groundwater Basin Coastal Monitor Wells - Since 1990, the District has been collecting water quality samples from coastal monitor wells in the Seaside Groundwater Basin, for the purposes of water quality characterization and sea-water intrusion monitoring.In Fall 2007, 12 dedicated monitor wells at six different sites were sampled.Results of water quality sampling from Fall 2007 and Fall 2006 for the Seaside wells are provided in Exhibit 27-A and Exhibit 27-B, respectively.The locations of the Seaside wells are shown on the map in Exhibit 27-D.These results indicate little change from previous results over the period of record for the existing wells, and that there is no indication of sea-water intrusion in these wells that are completed in the two principal aquifer units -- the Paso Robles Formation (i.e., shallower unit) and Santa Margarita Sandstone (i.e., deeper unit) -- in this area of the Seaside Groundwater Basin at the present time.A graph showing the historical period of record for SEC and chloride concentration at a representative pair of coastal monitor wells is presented in Exhibit 27-I.†† Results for SEC were mostly unchanged, that is, changed less than five percent relative to 2006, with two exceptions.SEC for wells 15S/1E-23Ca and 15S/1E-15K5 was close to 20 percent higher in 2007 relative to 2006.No remarkable changes were detected in other constituent concentrations for any wells in the area.Staff will continue to track results for trends that might indicate significant changes in any wells in the basin.†† A more complete historical summary of the Seaside Basin coastal groundwater quality data is contained in District Technical Memorandum 97-02 Seaside Basin Coastal Monitor Wells: Ground Water Quality Monitoring Results, 1990-1996, which is available at the District office.

 

EXHIBITS

27-A††† Ground Water Quality Monitoring Results - Fall 2007

27-B††† Ground Water Quality Monitoring Results - Fall 2006

27-C††† Location of MPWMD Carmel Valley Water Quality Monitoring Wells

27-D††† Location of MPWMD Seaside Basin Water Quality Monitoring Wells

27-E††† Water Quality Results in Well 16S/1W-14Jg in Carmel Valley

27-F†††† Water Quality Results in Well 16S/1W-13Lc in Carmel Valley

27-G††† Water Quality Results in Well 16S/1W-13Md in Carmel Valley

27-H †† Water Quality Results in Well 16S/1E-23La in Carmel Valley

27-I†††† Water Quality Results in Wells 15S/1E-15N2 and -15N3 in Seaside Coastal Subareas

 

 

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